Photosensitive cathodes



March 25, 1969 R, G, STQUDENHEMER ET Al. 3,434,876

PHOTOSENSITIVE CATHODES Filed NOV. 23, 1965 /1/ I f//f I fl 'l' /r I Y 30a@ 40a@ .000 6000 7000 am@ 9000 United States Patent O PHOTOSENSITIVE CATHODES Richard G. Stoudenheimer, Lancaster, and Daniel L.

Thoman, Rohrerstown, Pa., assignors to Radio Corporation ol' America, a corporation of Delaware Filed Nov. 23, 1965, Ser. No. 509,374 Int. Cl. Hk 3/16; HOSb 33/28; B44d 1/18 U.S. Cl. 117-211 '7 Claims ABSTRACT 0F THE DISCLOSURE The sensitivity and red response of a bialkali photocathode for a photoemissive electron tube are enhanced by providing a film of antimony oxide between the photocathode and a substrate on which the photocathode is formed.

This invention relates to photosurfaces used as photoemissive or photosensitive cathodes in such devices as image converter tubes, phototubes, photomultiplier tubes, camera tubes for television, or the like, and to methods for forming such photosurfaces.

lt is known (see for example U.S. Patents 2,770,561 for Photoelectric Cathode and Method of Producing Same, and 2,914,690 for Electron-Emitting Surfaces and Methods of Making Them) to form a photocathode by providing an electron-emitting surface, such as a iilm of antimony activated or sensitized with one or more alkali metals, on a substrate, such as glass. A thin film of potassium may be provided between the antimony film and the substrate.

Objects of this invention are to provide photosensitive or photoemissive cathodes of a type similar to those described in the aforementioned patents, but having improved sensitivity and improved response to red light, and to provide novel methods for forming said cathodes.

For achieving these objects, an intermediate lm of antimony oxide is provided between the antimony film and its substrate. The antimony oxide film is preferably obtained by evaporating antimony from an antimony-platinum source and oxidizing the evaporated layer.

In the drawing:

FIG. 1 is a sectional view of a device which can be used to provide a photocathode; and

FIG. 2 represents the spectral response of different photocathodes.

FIG. 1 discloses a device or tube 6 which, by way of example, can be used to form photoemissive surfaces. The tube 6 comprises an envelope 8 including an elongated bulb 10 and a stern 12 hermetically sealing the lower end of the bulb. Sealed through the stem 12 are a plurality of pairs of leads 14, 16, 18 and 20.

Between the enclosed ends of the lead pair 14 is mounted a iilament 22. The filament 22 com-prises, preferably, a platinum clad molybdenum wire or a platinum plated tungsten wire. Attached to the filament 22, as by being melted thereon (in a hydrogen atmosphere) is an antimony-platinum alloy pellet 23 comprising about 50% antimony and Vabout 50% platinum, by weight. Also mounted on filament 22 is a cone-shaped nonconductive shield 25. As described hereinafter, antimony from the pellet 23 is deposited onto the surface of the bulb 10. One purpose of the shield 25 is to confine the antimony to a preselected area at the upper end of the bulb.

Mounted between the enclosed ends of lead pairs 16, 18, and 20 are generators 26, 28, and 30, respectively, each generator being a hollow, elongated container, and being the source of an alkali material to be utilized in the formation of the photocathode surface. The wall of each generator is made of a material such as nickel, nichrome, tantalum, or the like, whereby upon the passage of an 3,434,876 Patented Mar. 25, 1969 ice electrical current through each lead pair, each generator may be heated.

The `generator 26 contains a mixture of potassium chromate, aluminum, and tungsten. This mixture, upon heating, chemically reacts to form a vapor of pure potassium metal. The generator 26 is vented to permit the escape of the vapor therefrom. The generator 28 contains a mixture of sodium chromate, aluminum, and tungsten which provides, when heated, a vapor of pure sodium metal. The generator 30 contains a mixture of cesium chromate and silicon which, when heated, provides a vapor of pure cesium metal. Another purpose of the shield 25 is to prevent direct-line evaporation of the alkali materials onto the photocathode surface. For reasons not fully understood, indirect evaporation paths provide better results.

A source of oxygen, not shown, is connected to the bulb 10 through a tubing 34.

An exhaust tubulation 40 is provided which communicates with the interior of the envelope 8 through an opening in the Vstem 12. The exhaust tubulation 40 is attached to an exhaust manifold and pumping system, not shown, whereby the tube envelope 8 may be evacuated.

In the top end of the bulb 10 is a conducting lead 44 which extends through and is sealed to the glass wall of the bulb.

Methods of forming a photosensitive or photoemissive surface on the inside wall of the upper end of the bulb are now described.

In the forming of a photosensitive surface on a substrate, comprising, for example, the inside wall of the upper end of the bulb 10, the tube envelope is rst evacuated until the pressure in the envelope is in the order of l0h6 mm. of mercury or less. A current is passed through the filament 22 by connecting the lead pair 14 to an appropriate voltage source (not shown) in order to heat the iilament to a temperature suiiiciently high to evaporate antimony from the antimony-platinum pellet 23. The platinum has a vapor pressure somewhat lower than the vapor pressure of antimony, hence is not evaporated with the antimony. The evaporated antimony deposited on the inner surface of the glass bulb 10 is confined to a film 48 of antimony on the upper end of the bulb by the shield 25. The film 48 of antimony makes electrical contact with the conducting lead 44 sealed through the glass wall of the bulb.

The evaporation of the antimony is continued until the light transmission through the film 48 is preferably between to 95% of the light passing through the bulb wall 10 prior tothe formation of the fil-m 48. Light transmission through the lm 48 can be measured in the manner disclosed in U.S. Patent 2,676,282.

The antimony tilm 48 is then exposed to an oxygen atmosphere by admitting oxygen through the tubing 34 into the bulb 10. Preferably, an oxygen pressure in the range of 10-1 to 3 l01 torr is provided within the bulb 10. For oxidizing the antimony lm 48, a glow discharge is provided within the bulb. This is accomplished by connecting the lead 44 to the negative terminal of a potential source 50 and connecting one of the leads 14 to the positive terminal of the potential source. The potential source 50 provides a D.C. voltage (when connected into the current by switch means, not shown) suicient to create a glow discharge through the oxygen atmosphere within the bulb. As a result of this glow discharge, the antimony film 48 is oxidized.

Another method of oxidizing the antimony film 48 is as follows: the tube 6 is heated until the envelope 8 and its contents are at a temperature of around C. to 230 C. Oxygen is then admitted into the envelope 8 through the tubing 34. The oxygen oxidizes the hot antimony layer 48 upon contact therewith.

It has been discovered that an antimony film evaporated from a source of pure antimony is not readily oxidized upon the performance of the above-described oxidizing steps. The antimony film may be readily oxidized, however, when the film is evaporated from an antimonyplatinum source, as described.

Having provided a substrate of antimony oxide, a photosensitive or photoemissive cathode is formed by providing a sensitized film of antimony on the film of antimony oxide. In general, the methods described in the aforementioned U.S. Patents 2,770,561 and 2,914,690, may be used with satisfactory results. The methods described hereinafter, where differing from the methods described in the patents, are found to provide somewhat better results.

A current is again passed through the filament 22 to evaporate a thin film 54 of antimony onto the antimony oxide film 48. The antimony film 54 is preferably formed while the tube 6 is at room temperature, and the formation of the antimony film 54 is continued until the light transmission through the films 48 and 54 is between 40 to 80% of the light passing through the antimony oxide film 48 prior to the formation of the film 54.

Potassium is then made to react with the antimony film 54. This is done by connecting the lead pair 16 to a potential source, not shown, and causing current to pass through the generator 26 to heat it and form the pure potassium metal. The potassium metal thus formed passes out of the generator 26 and into the envelope 8 and into contact with the antimony film S4. The potassium reaction is preferably carried out at a tube thermal equilibrium temperature of around 170 C.

The duration of this process is determined by monitoring the photoemission from the films 48 and 54 or, as referred to hereinafter, the surface 56. This is accomplished by making the surface 56 negative with respect to the leads 14 by means of the potential source 50, and shining light onto the surface. A microammeter 60 is connected in series in the circuit of the potential source 50 to measure any electron discharge between the surface 56 and the positive leads 14, which serve as an anode. The potassium reaction process is continued until the photosensitivity, as measured by the electron emission from the surface 56 reaches a peak, at which time the source of the potassium is removed by stopping the electrical current through the generator 26. The tube 6 is now baked at about 170 C. for a period of time until the photosensitivity reaches another peak.

The tube 6 is then heated to a thermal equilibrium temperature between 210 and 230 C., and sodium is made to react with the surface 56 on the bulb 10. The sodium is provided by passing an electrical current through the lead pair 18 to heat the generator 28. Within the envelope, the sodium reacts with the surface 56. This process is continued until the electron emission from the surface 56 first reaches a peak and then decreases by about 70-90% from the peak.

In contrast with known photocathodes, such as the cathodes described in the aforementioned U.S. Patent 2,770,561, it is found preferable to use an amount of sodium greater than was heretofore used. One means for providing the extra sodium is to use one or more sodiumantimony alternations. That is, sodium is evaporated onto the surface 56 until the electron emission reaches a peak. Antimony is then evaporated onto the surface S6 until the electron emission level is down 7090% from the sodium peak. This cycle is then repeated. Preferably 3 sodium-antimony alternations are used.

Subsequent to the sodium reaction process, a number of potassium-antimony alternations are performed. To accomplish this, the temperature of the tube is lowered to around 170 C. and current is successively passed through the lead pairs 16 and 14, respectively. The potassium evaporation step is first performed until the electron emission reaches a peak. The antimony evaporation step is then performed until the emission level is down by about 70-90% from the potassium peak. The antimonypotassium alternations are continued until the peak sensitivity obtained with the potassium addition ceases to be higher than the peak sensitivity of the previous alternation. As many as 8 to 20 alternations may be used.

Cesium is then made to react with the surface S6. The Cesium is obtained by passing an electrical current through the lead pair 20 for heating the generator 30. The pure Cesium metal thus produced reacts with the surface 56. The process is continued until the photocathode sensitivity reaches the maximum.

After the cesium reaction step, antimony is again evaporated from the filament 22 until the photocathode sensitivity is down by about 50% from the Cesium maximum. The Cesium reaction step is repeated until the photocathode sensitivity reaches a higher peak. Further alternations are continued until the peak sensitivity obtained with the cesium addition ceases to be higher than the peak sensitivity of the previous alternation.

The surface 56 on the bulb wall is now an ecient photocathode.

In another embodiment, a potassium layer or film is provided between the antimony oxide film 48 and the inside surface of the bulb i0. The potassium film may be provided as disclosed in the aforementioned U.S. Patent 2,914,690.

It is not evident what is the function of the several materials in the photoelectric phenomenon. However, the terms film and surface, used in the specification are not to be construed as necessarily implying physical continuity and homogeneity. It may be that the materials used form into spaced globules or even molecules and that they may also chemically combine with each other to form a discontinuity not possessed by the usual meaning of the terms used.

Photocathodes made as described are found to have increased spectral sensitivity throughout the yellow and red regions of the spectrum. FIG. 2 shows the spectral response curve 62 for a prior known photocathode made in accordance with the teaching of U.S. Patent 2,770,561, and the spectral response curve 64 for a photocathode including an antimony oxide film directly on the support surface as herein described. The ordinate of the graph is milliamperes per watt (ma/W.) and the abscissa is wavelength (W), measured in angstroms. Curve 64 shows a response at a wavelength of 8,000 angstroms which is about 20 times higher than the response at this wavelength shown by the curve 62. Also, for a sensitivity of one millimicroampere/watt, the curve 64 extends about 900 angstroms further into the infrared than the curve 62.

The spectral response of photocathodes made according to this invention is somewhat dependent upon the thickness of the initial antimony film 54 formed on the antimony oxide. In general, thicker initial lms, e.g., in the order of 40% light transmission, provide cathodes having a peak sensitivity further into the infrared range than cathodes having thinner initial layers, eg., in the order of light transmission.

Although the peak sensitivities of curves 62 and 64 are about the same, photocathodes as herein described have been fabricated having sensitivities generally higher than the sensitivity of the prior art photocathodes. That is, photocathodes as herein described have vconsistently yielded sensitivities of about 200 microamperes/lumen, and, in some instances, as high as 270 microamperes/ lumen. Prior art photocathodes have sensitivities in the order of microamperes/lumen.

A further advantage of photocathodes as herein described is that it has been noted that the sensitivity of the cathodes tend to increase with shelf life. That is, measurements of sensitivity made both before and after several months of storage often reveal au increase in cathode sensitivity.

What is claimed is:

1. A photoemissive cathode comprising a substrate, an antimony oxide lm on said substrate, and a photoernissive lrn containing antimony plus at least one alkali metal on said antimony oxide lm.

2. A cathode as in claim 1 wherein said substrate includes potassium.

3. In the method of forming a photosensitive cathode, the steps comprising evacuating an enclosure, vaporizing a film of antimony from an antimony-platinum source onto a substrate in said enclosure, introducing oxygen into said enclosure, oxidizing said antimony lrn, and forming a photoemissive ilm containing antimony on said oxidized antimony film.

4. The method of claim 3 wherein said vaporizing step is continued until the light transmission through said substrate is reduced to about 80 to 95% of what it was prior to said vaporizing step.

5. The method of claim 3 including the step of providing a film of potassium on said substrate prior to said antimony vaporizing step.

6. The method of forming a photosensitive cathode in an enclosure comprising:

evacuating said enclosure,

providing a surface of antimony by evaporating antimony from an antimony-platinum source in said evacuated enclosure,

introducing oxygen into said evacuated enclosure, oxidizing said antimony surface, evaporating potassium onto said surface, alternately evaporating sodium and antimony onto said surface, alternately evaporating potassium and antimony to said surface, and alternately evaporating cesium and antimony onto said surface. 7. The method of claim 6 wherein a film of potassium is provided on said substrate prior to the antimony evaporating step and after the evacuating step.

References Cited UNITED STATES PATE-NTS 2,244,720 6/1941 Massa et al. 117-107 2,574,356 11/1951 Sommer 117 -225 X 2,770,561 11/1956 Sommer 117-211 2,880,344 3/ 1959 Stoudenheimer 117-34 2,914,690 11/1959 Sommer 117-211 3,179,835 4/1965 Kasernan 117-219 X ALFRED L. LEAVITT, Primary Examiner. A. GOLIAN, Assistant Examiner.A

Us. C1. XR. 117-34, 93.1, 106, 107, 215, 217, 219, 224; 313-65 

